This invention relates generally to prodders for probing the ground for buried explosive devices such as landmines and the like, and more particularly to a method and device for providing force feedback to the prodder and/or the user of the device.
Despite a variety of mechanized means now available for detecting and clearing landmines, the current hand tool of choice is the hand prodder. Personnel exhibit greater confidence when traversing a minefield which has been hand-prodded by their compatriots than they do with fields cleared by other means.
The traditional hand prodder typically comprises a 30 cm long pointed rod extending from a gripping handle. The probe is generally non-magnetic to avoid setting off magnetically-triggered mines. The user probes the ground ahead and excavates any hard objects which the probe contacts. As the ratio of rocks to landmines in a minefield may number 1000:1, excavation of every contact is labourious, but very necessary.
Currently, instrumented prodders are known having ultrasonic means in the form of an ultrasonic transducer at or near the probe tip that are used for characterization of buried obstructions. These devices can be used in conjuction with a minimum metal content (MMC) detector, wherein the MMC detector first detects the ground indicating the vicinity of a land mine, and, wherein the instrumented prodder is used to probe the earth in the vicinity of the suspected land mine, the location of which may have been isolated using the MMC detector. MMC mine detectors having a search head and circuitry for detecting buried non-metallic and metallic land mines are well known. For example, U.S. Pat. No. 4,016,486 in the name of Pecori as signed to the United States of America by the Secretary of the Army, hereby incorporated by reference, discloses such circuitry.
U.S Pat. No. 5,920,520 to Gallagher, hereby incorporated by reference, discloses an instrumented prodder having a probe in the form of an elongate, preferably non-magnetic rod including a gripping handle disposed at one end. The design of the probe is based partially upon a Split Hopkinson Pressure Bar (SHPB) apparatus. In the apparatus, a compression wave or high frequency elastic mechanical pulse is delivered via a rod to a sample, wherein a portion of the wave is reflected. The incident wave launched at the sample is reflected and/or transmitted from or through the sample, respectively, in dependence upon the characteristics of the material. The effect of mechanical impedance, which is a characteristic of a material, on a SHPB apparatus in three instances is described hereafter:
Firstly and obviously, if the mechanical impedance of a sample under test is the same as that of an incident bar in the SHPB, there will be no reflection as the sample will be displaced in a same manner as the bar itself as the compression wave is delivered. The displacement of the end of the bar is directly proportional to the strain measured (xcex5).
Secondly when the mechanical impedance of a sample is considerably greater than that of the bar, a sample""s mechanical impedance tends toward being infinite and substantially the entire wave is reflected.
In a third instance when the mechanical impedance is zero, in the absence of a sample, the reflected wave is tensile but of equal magnitude to the incident wave. The phase of the wave is shifted by xcfx80 and the net stress is zero; the relative displacement at the bar end equals twice that for the first instance (2xcex5).
In a SHPB device, once the relative displacement of the bars is known, the displacement of the sample is ascertained. Taking into account Young""s Modulus (E) and the displacement of the bar, the imposed stress can be calculated, wherein the force applied is equal to the product of the stress and the cross-sectional area of the bar.
Since the loading on the sample becomes equal after a short time, the analysis may be somewhat simplified. Strain results may be used for only the incident bar; or alternatively, the striker bar may be directed to impact directly on the sample, and the transmitter bar alone may be used to define the sample characteristics.
It is has been found that plastics, minerals and metals may be discerned from one another by using this approach.
It has been further found that the hand held prodder disclosed by Gallagher having a rod modified to be analogous to the incident bar of a SHPB may be used to detect or discern metal, plastic and rocks.
The prodder rod is provided with one or more piezoelectric transducers capable of generating an acoustic wave into the rod and for detecting reflected waves from an object contacting the end of the rod. Conveniently, signal processing means are coupled to the transducers and are provided for analysing the detected reflected waves for determining the characteristics of the object; more especially, for distinguishing landmines from inert rocks. The signal processor establishes measurements of the frequency-time-amplitude characteristic of the object. The reflected waves are compared with known characteristic signatures of a plurality of materials to attempt to ascertain a match within predetermined limits.
Although U.S. Pat. No. 5,920,520 describes a device that performs satisfactorily in many instances, it suffers from a problem related to the fact that acoustic coupling at the obstruction is a function of the force applied to the probe end. As a result, the results are often erroneous. This is partic ularly detrimental when the prodder indicates that the obstruction is a rock, when in fact it is a land mine.
Preferably, enough force will be applied to the probe end such that characterisation of the obstruction can occur without causing detonation; and, preferably, a relatively consistent force will be applied to the probe end such that an accurate determination as to the character of the buried obstruction can be made. However if too little force is applied at the probe end, a poor reading may result and a mine in the vicinity of the probe may go undetected. Too much force applied at the probe end in the vicinity of a land mine may inadvertently detonate the mine.
In prior art FIG. 1 a specimen sample is shown juxtaposed between an incident bar and a transmitter bar. A strain gauge disposed on each bar provides a signal-to-signal processor as is described heretofore.
In prior art FIG. 2 a hand-held prodder for probing the ground for buried explosive devices such as landmines and the like is provided. The prodder comprises a rod 2 having a first end 3 flexibly supported by an annular rubber coupling 4 in a mounting nub 5. The nub 5 is screwed into a handle 6. The rod has a pointed second end 7 for sensing objects 8 buried in the ground 9.
The rod 2 is 45 cm long and is formed of non-magnetic, austenitic stainless steel. Only 30 cm project from the rubber coupling 4. The rubber coupling 4 lessens the rigidity between the rod 2 and handle 6.
Best seen in prior art FIG. 3, a piezoelectric crystal 10 is glued to the first, or driver end 3 of the rod 2. When an electric field is applied to the crystal 10, a mechanical strain will occur and drive mechanical energy into the rod""s driver end 3. Conversely, when the crystal 10 is mechanically stressed, an electric charge is produced. A suitable crystal is a 15 mm long, 6.35 mm diameter poly-crystalline ceramic cylinder, model Sonex P-41 available from Hoechst CeramTec, Mansfield, Mass. The crystal 10 is electrically insulated from the rod 2 with a ceramic insulator 11. Optionally, the insulator further serves to provide mechanical strength to the joint between the crystal and the rod.
Positive and negative electrical leads 12 from the crystal pass through the nub 5 for bidirectional electrical signal transmission between the crystal 10 and an electronics module 13. Shown in FIG. 2, the module 13 is installed within the prodder""s handle and is powered with 9 V batteries 14.
The electronics module 13 is capable of two modes: a driver mode and a signal processing mode. In the driver mode, an electrical signal is transmitted along leads 12 to the crystal 10 for generating a piezoelectric mechanical pulse. The pulse is introduced into the rod""s driver end 12. In the signal processing mode, any electrical signals generated by the crystal 10 are transmitted along leads 12 for processing by the electronics module 13.
More specifically, the module 13 comprises a digital signal processing microcomputer 15, an EPROM 16 containing program instructions and digital storage means, an A/D converter 17, a signal input amplifier 18 and a driver output amplifier 19. An audio/visual binary output device 20 is provided.
A suitable signal processor is a model ADSP-2181 digital signal processing microcomputer by Analog Devices, Inc., Norwood, Mass. The ADSP-2181 contains a high speed serial port, 16 bit data processing capabilities and has both onboard program RAM and data memory RAM. For permitting battery powered operation, the ADSP-2181 features a power saving xe2x80x9csleepxe2x80x9d mode. After downloading of program instructions from the EPROM, the ADSP-2181 will reduce its power consumption and await a suitable trigger before xe2x80x9cwaking-upxe2x80x9d to begin signal processing.
Having reference to the prior art flow chart in FIG. 4, when the prodder is activated, the EPROM 16 downloads the analysis program to the ADSP-2181 processor 15 and awaits a trigger. When triggered (i.e., contact of the rod""s sensing end with an object) the EPROM 16 signals the driver output amplifier 19 to generate an ultrasonic analog driver signal (20-200 kHz). The driver signal stimulates the crystal 10 to generate a mechanical pulse and send it as an acoustic incident wave down the longitudinal axis of the rod 2. The incident wave reflects from the object 8 at the rod""s sensing end 7 and returns to the rod""s driver end 3 as a reflected wave. The mechanical energy in the reflected wave stimulates the crystal 10 to generate electrical analog signals characteristic of the reflected wave.
The analog signals are processed through the signal input amplifier 18 and converted by the A/D converter 17 for analysis by the signal processor 15. A suitable A/D converter is available as model AD876 10 bit, 20 MSPS (million samples per second) CMOS converter, also from Analog Devices, Inc. The AD876 is also capable of a xe2x80x9csleepxe2x80x9d mode.
The digital processor 15 stores the reflected data in its RAM memory. The characteristics of the reflected signal are dependent upon the material characteristics of the object 8. Different materials have different mechanical impedance (MI) and frequency-dependent damping coefficients. Analysis of the reflections and damping rates demonstrated in the reflected data is instructive of the material characteristics of the object.
Accordingly, using one analytical technique, the stored data is conditioned using a stepping FFT and analysed for frequency-time-amplitude information. A 256 point FFT from a 1024 sample is advanced in 128 sample steps which yields 7 time-slices of FF transformed data. The characteristics distinctive of the material are generally located within the first 5-10 harmonics or bins of the transformed data.
The effects of the peculiar characteristics of the rod are calibrated by causing the piezoelectric crystal to send a pulse along the rod when its sensing end is not contacting anything. This xe2x80x9cdryfirexe2x80x9d provides a baseline reading which accounts for individual characteristics including the tapered point of the bar, wear, temperature, and accumulated debris. This resulting baseline power data is subtracted from the actual contact data.
Non-contact calibration can be done before each use to account for physical prodder variations. The extraction of the baseline rod characteristics heightens the sensitivity of the signal analysis, having removed a portion of the signal which is not attributable to the object.
However, non-contact calibration does not account for variations in pressure with which the sensing end 7 of the rod is forced against the object 8 to be detected. In fact, there is no attempt to calibrate the prodder with respect to effects from an applied force, such as pressure. This is a significant limitation to the prodder described heretofore and shown in the figures. Since readings acquired with the prodder are dependent upon applied pressure, and since the applied pressure is likely different each time the prodder is used, it is desirable to provide means for providing force feedback to compensate the readings for an applied force. Providing force feedback to the prodder and/or the user of the prodder also allows the applied force to be determined to calculate whether too little or too much force is applied to the object being detected. Preferably, the means for providing force feedback does not reduce the durability of the prodder and/or significantly increase the manufacturing cost.
Mechanical force sensors such as springs suffer from several disadvantages. Firstly, they are subject to fatigue over time. Secondly, they are often difficult to design such that they are robust enough and accurate enough for military applications without incurring significant costs. These drawbacks are well known and overcoming them would be advantageous.
It is therefore an object of the invention to provide a method and device, which will overcome the aforementioned problems, related to too much force, too little force, or a varying force being applied to the probe end while in use.
It is a further object of the invention to provide an instrumented prodder for detection of land mines and the like that includes force feedback for sensing a force, such as pressure, applied to an end thereof.
It is another object of the invention to provide an instrumented prodder for detection of land mines and the like, that provides data related to characteristics of the probed object that are independent from the force of the prodder on the object.
It is a further object of the invention to provide a hand-held prodder for probing the ground for buried explosive devices such as landmines and the like, that is relatively simple, rugged, and inexpensive.
In accordance with the invention there is provided a prodder having force feedback for detecting detonatable devices or land mines, comprising:
a rod having an end for placing in contact with a first object to be detected;
a first transducer coupled to the rod for providing an acoustic wave to the first object and for receiving acoustic waves reflected from about the first object;
a second rod having an end for placing in contact with a reference object;
a second transducer coupled to the second rod for providing an acoustic wave to the reference object and for receiving acoustic waves reflected from about the second object; and,
an electronics module for analysing data related to the waves reflected from the reference object to determine a value related to the force applied to the first rod, for analysing data related to the waves reflected from the first object in dependence upon the determined value to determine acoustic characteristics of the first object material so as to categorize the first object""s material in a force independent fashion.
In accordance with another embodiment of the invention there is provided a land mine detector having force feedback comprising:
a rod having a sensing end for contacting objects buried beneath the ground;
a transducer coupled to a non-sensing end of the rod for imparting mechanical energy into the rod towards the sensing end in contact with a buried object, and for detecting a signal corresponding to reflected waves from the buried object, the signal containing information related to a force exerted on the rod and to material characteristics of the buried object;
a second transducer coupled to an end of a second rod for imparting mechanical energy into the second rod towards another end in contact with an object having known characteristics, and for detecting a signal corresponding to reflected waves from the second object, the signal containing information related to the force exerted on the rod and to material characteristics of the known object; and,
means for processing the signals, for determining at least one of the force exerted on the rod, and force-independent material characteristics of the buried object for comparing with stored material characteristics of known objects so as to categorize the object""s material.
In accordance with yet another embodiment of the invention there is provided a prodder having force feedback for detecting detonatable devices or land mines, comprising:
a rod having an end for placing in contact with a first object to be detected;
a first transducer coupled to the rod for providing a first acoustic wave to the first object and for receiving first acoustic waves reflected from about the first object and for providing a second acoustic wave to the first object and for receiving second acoustic waves reflected from about the first object; and,
an electronics module for analysing data related to the first and second acoustic waves reflected from the first object to determine acoustic characteristics of the first object material so as to categorize the first object""s material in a force independent fashion.